Given that Debian's standard installers have always been pretty bad at configuring any non-trivial disk set-up without manual intervention, I feel some people here are a little too quick to criticise. As I said in another post, I don't know why the systems where that issue came up were originally set up as they were, but there have certainly been times, particularly before the current generation of bootloaders, when that sort of set-up wasn't unusual.
The point remains that this doesn't matter. Before the upgrade, there was a fully working system. After the automated part of the upgrade, there wasn't. The original question was how safe the upgrade from 7 to 8 is, and this is a demonstration of the fact that such upgrades can carry risk. I'm not saying don't do them, I'm not expecting Debian maintainers to be omniscient, and I'm not telling you your child isn't beautiful. I'm just saying if you're thinking about moving from 7 to 8, be aware of the potential that there will be things the automated tools can't or won't do for you that may break your system, and plan your upgrade or other migration strategy accordingly.